1. Field of the Invention
The present invention relates to apparatus and methods for measuring blood pressure and, in particular, relates to apparatus and methods for determining systolic, diastolic and mean arterial pressures by employing oscillometric techniques and analyzing electrical signals detected by a pressure transducer representative of pressures.
2. Description of Related Art
It is relatively well known in the art of blood pressure measurement design that the orderly depolarization of the heart muscle triggers a wave of contraction which "spreads" through the walls of the chamber of the heart containing the musculature that acts during the pumping of blood, i.e. the myocardium. The contraction process actually produces the pumping action, by sequentially changing the pressure (and, therefore, blood flow) in the heart chambers and blood vessels. However, primarily because of the elastic properties of the blood vessels in the arterial system, and the discontinuous nature of myocardial contraction which produces ejection of blood from the ventricles, the precise relationship between arterial blood pressure and blood flow is relatively complex. In addition, the discontinuous pumping action of the heart is opposed by the impedance of the arterial system.
The radial stretch of the aorta resulting from left ventricular ejection initiates a pressure "wave" which propagates down the aorta and the arterial blood vessels. The arterial pressure waveform becomes relatively progressively more distorted as the wave is transmitted down the arterial system. This distortion may be attributed to the impedance of the arterial system. The impedance of the arterial system can be affected by a number of factors, including: (1) characteristics of blood; (2) the viscoelastic characteristics of the arteries; (3) arterial capacitance; (4) "tapering" of the lumen of blood vessels; (5) resonance; (6) reflected pressure waves due to arterial branching, and changes in transmission velocity with pressure. Notwithstanding the foregoing, arterial blood pressure is a quantitative measurement routinely obtained in a variety of known ways for purposes of diagnosis of patients.
Arterial blood pressure is generally expressed in terms of systolic and diastolic pressures. These pressures are, respectively, upper and lower limits of period oscillations about a mean arterial blood pressure. Systolic pressure is typically measured when, during left ventricular ejection, a volume of blood is relatively rapidly introduced into the arterial system, initially into the aorta. A "maximum" arterial blood volume is reached at the end of this rapid ejection phase (commonly referred to as the early part of systole) which corresponds to a peak pressure defined as systolic pressure. Correspondingly, diastolic pressure is typically measured in the absence of ventricular ejection of blood; that is, when all four chambers of the heart are relaxed (i.e. diastole). This measurement typically occurs immediately prior to the next ventricular ejection. The lowest pressure obtained during diastole is characteristically defined as diastolic pressure.
As a graphical illustration of the events which occur in the heart and relate to blood pressure, FIG. 1 depicts certain blood pressure-related waveforms as a function of time. Referring specifically to FIG. 1, changes in aortic pressure are represented by aortic pressure waveform 100. As apparent from waveform 100, during ventricular systole, the ejection of blood from the left ventricle is initially relatively rapid. As the rate of pressure change decreases during systole, the maximum aortic pressure is obtained. Correspondingly, when the systolic period is completed, the aortic valve is closed by the back pressure of blood against the valve. This effect is illustrated in FIG. 1 as the dicrotic notch 102 of the aortic pressure waveform 100.
Also illustrated in FIG. 1 is arterial blood pressure waveform 104 representative of the blood pressure within a peripheral artery. With respect to the blood pressure waveform 104, a dicrotic notch 106 is illustrated, corresponding to the effect on the arterial blood pressure waveform of the completion of the systolic period resulting from the closing of the aortic valve by the blood back pressure against the valve. When the aortic valve is completely closed, the aortic pressure gradually decreases as blood pours into the arterial system, thereby producing blood pressure pulses, such as arterial blood pressure pulse 108. Correspondingly, the left ventricular pressure, illustrated in FIG. 1 as left ventricular pressure waveform 110, also gradually decreases until the left ventricle again begins to fill with blood in preparation for the next systolic period. This action will result in the arterial blood pressure pulse illustrated as arterial blood pressure pulse 112 on the wave form 104. The overall shape of each arterial blood pressure pulse will change, as each pulse passes through the arterial system. However, a dicrotic notch will always be present with respect to the pulses.
A number of devices are relatively well known for obtaining blood pressure measurements of a test subject. The most common form of blood pressure measurement employs indirect measurement techniques utilizing a compression bag or "pressure cuff" for the application of external pressure to an artery. Typically, this cuff is a pneumatic cuff adapted to encircle the upper arm of a test subject.
The most common form of blood pressure measuring device also employs a hand pump and pressure gauge. These devices are often referred to as sphygmomanometers. With these devices, an inflatable section of the cuff is inflated by a small hand pump and the cuff pressure is indicated by a mechanical pressure gauge or a manometer calibrated in terms of millimeters of mercury. The cuff is typically inflated to a pressure greater than the systolic blood pressure in the large brachial artery of the arm. This pressure essentially "collapses" a segment of the artery under the cuff, and occludes the blood flow through the artery.
After cuff inflation, the pressure in the cuff is typically allowed to decrease. This decrease is often achieved through the use of a release valve built into the hand pump. During pressure decrease, a pressure level is reached where the cuff pressure and the peak or systolic arterial pressure are substantially equal. At a pressure slightly below this cuff pressure level, the peak arterial pressure slightly exceeds the cuff pressure and blood first begins to flow or "squirt" through the compressed segment of the brachial artery.
This flowing or squirting blood will result in turbulence within the brachial artery, thereby creating sounds referred to as Korotkoff sounds. Auscultatory techniques are usually employed to determine the systolic and diastolic pressures based on the Korotkoff sounds. For example, it is typical to detect these sounds with a stethoscope placed over the brachial artery distal to the cuff.
As the pressure in the cuff is further decreased, Korotkoff sounds continue until a point is reached where no further turbulence is produced, since the brachial artery segment may then be just slightly constricted. This cuff pressure represents the diastolic blood pressure.
Although this technique, employing auscultatory methods for detecting Korotkoff sounds, is the most common type of blood pressure device currently available, these types of devices suffer from several disadvantages. Of primary importance, it is relatively difficult to detect the pressure where the Korotkoff sounds begin and where they cease, due to the interpretative nature of the measure. Accordingly, it is difficult, using auscultatory techniques, to provide an accuracy greater than approximately five millimeters of mercury.
Various techniques are also known for automating auscultatory devices. For example, the hand pump of the typical device may be replaced with an automatic cuff pump. The automatic cuff pump may be operated by utilizing a panel-mounted button to produce a single cycle of inflation and deflation or, alternatively, repetitive cycles at various intervals for continuous monitoring of blood pressure over relatively long periods of time may be provided. In addition, the stethoscope may be replaced by a Korotkoff-sound microphone, consisting of a relatively small piezoelectric transducer specially designed for accurate reproduction of Korotkoff sounds. Still further, the pressure gauge may also be replaced with a pressure transducer, similar to transducers utilized for direct measurement of blood pressure.
For providing further automation, the indirect blood pressure may be recorded, utilizing electronic apparatus, thereby somewhat increasing the accuracy of the auscultatory method. That is, such recording will somewhat remove human judgment in determining the presence of the Korotkoff sounds.
Further, the auscultatory techniques are relatively susceptible to false indications caused, for instance, by motion artifacts and ambient noise. In addition, the cuff must be substantially at heart level, to obtain a pressure relatively uninfluenced by gravity. Other problems can exist with respect to the actual physiology of the test subjects. For example, with individual test subjects having relatively obese arms, the body fat in the arm can somewhat dissipate some of the cuff pressure, such that pressure measurements are erroneously high. Still further, if the cuff is left inflated for some time, discomfort may cause generalized reflex vasoconstriction, thereby raising the blood pressure.
Still further, if an electronic unit is utilized to interpret output from a pressure transducer, correct positioning of the sound microphone over the brachial artery is critical. In addition, it has been found, based on direct and indirect blood pressure measurements made simultaneously, that diastolic pressure tends to correlate better with the cuff pressure at which the sounds become muffled, rather than with the pressure at which the sounds disappear (i.e. when turbulence ceases). Accordingly, the particular cuff pressure at which Korotkoff sounds tend to cease does not appear to be an important criteria in determining the actual diastolic blood pressure.
Various types of blood pressure measurement devices employing auscultatory techniques are shown in a number of patent references, including: Squires et al, U.S. Pat. No. 4,216,779; Gemelke, U.S. Pat. No. 4,252,127; Uemura, U.S. Pat. No. 4,356,827; Peterson, U.S. Pat. No. 4,378,807; Ichinomiya et al, U.S. Pat. No. 4,417,587; Sainomoto et al, U.S. Pat. No. 4,592,366; and Peel et al, U.S. Pat. No. 4,617,937.
Another method employed for blood pressure measurement techniques, and the method most relevant to the present invention, is typically referred to as an oscillometric or oscillatory method. Oscillometric blood pressure measuring devices essentially comprise analysis of electrical signals related to the blood pressure waveform. In many oscillometric devices, a pneumatic cuff is implemented substantially as previously described with respect to devices employing auscultatory techniques. However, in most oscillometric methods, a single pressure transducer is employed to detect both cuff pressures and pulsating pressures, resulting from periodic changes in pressure in the blood vessel. These pressure transducers convert the pressure detections into proportional electrical signals. These signals are then analyzed and processed for determining, from the pulsating pressures, the cuff pressures corresponding to the systolic and diastolic blood pressures. The signals representative of the pulsating pressures can also be utilized for determining other blood pressure characteristics, such as the mean arterial pressure.
A substantial number of oscillometric blood pressure measuring systems are currently known. A number of these systems comprise oscillometric blood pressure measuring devices utilizing a pressure transducer for converting cuff and pulse pressures into electrical signals. Various hardware circuitry and/or computer software can be employed to analyze "characteristics" of the pulses. For example, various known systems employ electrical and electronic devices for analyzing characteristics such as pulse height, pulse width and the like. Many of the inventions claimed in patent references directed to oscillometric systems are specifically directed to the pulse analysis techniques for making a determination of the relevant blood pressures.
With respect to the various known oscillometric blood pressure measuring systems, several potential problems are known. For example, blood pressure pulses superimposed on the occluding pressures are typically relatively small in height. Accordingly, it is somewhat difficult to detect the pulsating pressures within signals representative of the cuff pressures. Further, even relatively slight movements of the test subject during a measurement cycle can result in "false" pulse representations within the transducer output signals. Such false pulses are often characterized as "artifacts." Such pulses artifacts can be caused not only by the test subject's movement of the arm comprising the measured artery, but can also be caused merely by the slight movements associated with respiration. A number of other reasons also exist for artifact generation.
Again, the disadvantages associated with artifact generations and oscillometric measuring systems are relatively well known. The pulse analysis techniques described in many patent references are specifically directed to discriminating between true pressure pulsations and artifact pulse representations, and minimizing erroneous blood pressure determinations resulting from the artifacts.
An example of a blood pressure measuring device employing oscillometric techniques for determining systolic and diastolic blood pressures is disclosed in the U.S. Pat. No. 4,407,297 issued to Croslin and dated Oct. 4, 1983. As disclosed in the Croslin patent, after an artery is occluded by inflating a pneumatic cuff in the usual manner, four successive pulses must be detected as the pressure in the cuff is permitted to decrease at a rate less than 10 mm HG between successive pulses. That is, an error is considered to have occurred if the difference between a pulse and the preceding pulse is at least 10 mm HG. Also, an error is considered to have occurred if more or less than four pulses are detected when the cuff pressure deflates to 30 mm HG. For each detected pulse, two data items characterizing each pulse are stored in a table in memory. The first data item stored for each pulse is the cuff pressure detected at the start of the pulse. A second data item is the maximum amplitude of the pulse. A pulse is assumed to have occurred when a maximum amplitude is at least equivalent to 1 mm HG. However, when the value of the pulse amplitude is greater than a predetermined value, the pulse is assumed to be too large, and the measurement cycle is aborted.
Pulses are counted within a 10-second time interval, beginning with the detection of the third pulse. A 3-second time interval begins after the 10-second time interval times out. At least one pulse must be detected during the 3-second interval. If the sequence of pulses is considered valid, the cuff pressure detected at the start of a predetermined pulse is characterized as the systolic pressure. Diastolic pressure is determined to have occurred when the average of the amplitudes of four pulses is less than a predetermined threshold value, calculated as a function of the detected mean arterial pressure. In summary, if the amplitude of a pulse exceeds the previous pulse amplitude by more than 1 mm HG, or if the cuff pressure has decreased by at least 10 mm HG between successive pulses, an artifact is assumed to have been detected, unless four successive pulses have been previously detected for determining systolic blood pressure.
In another arrangement, as disclosed in U.S. Pat. No. 4,263,918 issued to Swearingen et al and dated Apr. 28, 1981, a somewhat different pulse analysis is undertaken. Again, a transducer provides a signal which is separated into a signal representative of static cuff pressure and a signal representative of pulsatile pressure in the cuff resulting in changes in arterial blood pressure. A comparator determines the start of each pulsatile signal, and produces a control signal when the pulse amplitude exceeds a predetermined reference value. The cuff pressures associated with the pulsatile signals are stored in a cuff pressure table in digital memory. Six of the "highest amplitude" pulses are also stored in a pulse table memory.
Of the six stored pulse amplitudes, the three pulses having the largest amplitudes, although not necessarily adjacent to one another in the table, are selected. That is, the pulse having a maximum height is selected, as well as two other pulses closest thereto in amplitude.
The Swearingen et al technique utilizes an algorithm whereby the pulses in the table are examined until a three-pulse set is detected, wherein the pulse having maximum height is less than or equal to 125 percent of the smallest of the other two pulses. When detected, the average of these three largest amplitude pulses is utilized as a reference level.
For purposes of obtaining systolic blood pressure, a level of 45 percent of the reference level is utilized. The system then searches upward in the pulse table, in the direction of higher cuff pressures from one of the six highest amplitude pulses, until another set of three pulses is found whereby two of these three pulses are each less than the systolic threshold level, and the third of these three pulses is greater than or equal to the systolic threshold level. The third pulse must be detected adjacent to the other two pulses, but closest to the single-highest amplitude pulse from which the search was initiated. The address of this third pulse in the pulse table is utilized as a pointer to the corresponding cuff pressure in the cuff pressure table. This cuff pressure is characterized as the systolic blood pressure.
For purposes of obtaining diastolic blood pressure, a diastolic threshold level is determined as 75 percent of the reference level. The algorithm then searches downward in the pulse table in the direction of lower cuff pressures, from the same one of the six highest amplitude pulses until another set of three pulses is found, whereby two of these three pulses are each less than the diastolic threshold level, and the third of these three pulses is greater than or equal to the diastolic threshold level. This third pulse must be adjacent to the other pulses, but closest to the one highest amplitude pulse from which the search was initiated. The address of this third pulse in the pulse table is utilized as a pointer to the corresponding cuff pressure in the cuff pressure table. This cuff pressure is characterized as the diastolic blood pressure.
In the Swearingen et al system, an error is considered to have occurred if the systolic pressure is not greater than the sum of diastolic pressure, plus 10 mm HG. Each of the cuff pressures is determined by subtracting a zero pressure offset, representing at least 2 mm HG, from static cuff pressure. For purposes of attempting to insure accuracy, a minimum number of 16 pulses must be obtained, at a cuff "bleed" rate of 3 to 6 mm HG per beat. At least 10 pulse amplitudes must be stored in the pulse table (in the direction of lower cuff pressure) beyond the predetermined highest amplitude pulse from which the previously described searches were initiated.
The ten pulses are utilized to assure that pulses are available after the occurrence of any "gap." To further improve accuracy, the concept is utilized that if the lowest amplitude pulse in the group of six highest amplitude pulses is less than or equal to a predetermined number, or greater than the predetermined number and less than or equal to a second predetermined number, or otherwise greater than the second predetermined number, a "stop measurement" cuff pressure is provided at either 75 mm HG, 85 mm HG or 120 mm HG, respectively. Accordingly, if the very last pulse table is greater than the applicable "stop measurement" cuff pressure, additional pulse pressures are acquired by the system. Otherwise, the worst case peak average reference is obtained from the three lowest pulse amplitudes of the set of six highest pulse amplitudes, and is characterized as 25 percent of the sum of the three lowest of the six highest pulse amplitudes. This worst case peak average reference would be less than or equal to any diastolic threshold level then currently determined. If two pulses are found by the system (in the manner previously described with respect to obtaining diastolic pressure) having amplitudes less than the worst case peak average, diastolic blood pressure is obtained. If two such pulses cannot be found, the system will acquire additional data, but the next pulse must be acquired within 1.8 seconds.
For purposes of discriminating true pulses from artifacts, the pulse amplitudes, in the pulse table, next adjacent or two away (on either side) from the largest pulse must be greater than or equal to 66 percent of the largest pulse. If artifacts are detected, the system continues to find the set of three highest pulses and the set of six largest pulses in the pulse table, meeting the appropriate criteria.
Again, a primary emphasis of a number of the patent references directed to oscillometric blood pressure measuring devices is the discrimination of artifacts versus true pulses, and resulting minimization of erroneous blood pressure measurements. As also previously described, various pulse characteristics are employed by several known systems, for purposes of accurately determining the relevant blood pressure levels. Although a number of these references primarily concentrate on characteristics such as pulse height, other characteristics can also be analyzed.
For example, U.S. Pat. No. 4,751,930, issued to Terada et al and dated June 21, 1988, is directed a pulse analysis arrangement utilizing pulse characteristics derived as functions of the pulse integrals. As disclosed, the Terada et al system includes a conventional occluding cuff coupled through a tube to a pressure transducer. The pressure transducer is mounted to an apparatus having associated electronics. The electronics include a low-pass filter, analog/digital (A/D) converter and microcomputer. Resultant blood pressure measurements are provided to a display.
The output signal from the transducer, representing both the "static" cuff pressure and the arterial blood pressure, is applied as an input signal to the A/D converter. The A/D converter samples the analog signal for providing discrete digital signals applied to the microcomputer. The microcomputer includes software functions for extracting, from the sampled cuff pressure data, a "pulsating quantity" representative of each blood pressure pulse occurring in sequence. Also included in the functions of the microcomputer is the feature of extracting the static cuff pressure from the sampled cuff pressure data.
In the Terada et al patent, the "pulsating quantity" is specifically described as being an integral or a function of an integral of the blood pressure pulse superimposed upon the static cuff pressure. For purposes of deriving this pulsating quantity, the Terada et al system measures data which includes the pressure level at the onset of a pulse. Also measured is the time of the onset of the pulse, the pressure level at the peak of the pulse, and the time of occurrence of the pulse peak.
For purposes of determining the pressure level at the onset of the pulse, the microcomputer senses when the actual pressure increases relative to the pressure measured during the last time interval. If such an increase occurs, and if such an increase is maintained over a predetermined time interval, the pressure level at the onset of the pulse is assumed to have occurred. Correspondingly, the time of occurrence is also obtained. The peak of the pulse pressure, in addition to the time of occurrence of the peak, is obtained in a similar manner, i.e. by determining when the increase in cuff pressure begins to decrease.
The Terada et al patent also discloses various arrangements for obtaining the pulsating quantity in terms of the integral of the pulse. One such arrangement is a computation of the total area under the pulse "curve" above a base pressure at the onset of the pulse. This integral value is obtained for each of the pulses, and is essentially "plotted" with respect to time. Correspondingly, the microcomputer also keeps track of the static cuff pressures at the onset of the pulses.
In another method described in the Terada et al patent, instead of obtaining the total area under the pulse curve above the onset base pressure for each pulse, the pulsating quantity can be computed as the area of the region bounded by the pulse curve between the onset time and the peak time, above the onset base pressure level. That is, the area of the pulse utilized is only that area above the onset base pressure level and up until the time of the pulse peak. This arrangement is described as being preferable, since the complutation of the integral of the pulse beyond the pulse peak may fluctuate, depending upon the bleed rate of the cuff pressure. This type of integral measurement is further characterized within the patent as providing a more consistent analysis of the pulses, resulting in relatively more reliable determinations of the blood pressure measurements.
Further, the Terada et al patent additionally describes the concept that the pulsating quantity can be defined as a quotient of an integral of the pulse, divided by a particular value of time within the time period of the pulse. In other words, the pulsating quantity would be an average pressure level during a pulse (above the onset value) obtained by dividing the integral of the pulse by the particular time width.
In the Terada et al system, and a number of other oscillometric blood pressure measuring systems currently known, sampling of instantaneous cuff pressures typically is performed at a rate which is substantially more frequent than the rate of blood pressure pulses. In addition, within such systems, sampling is continuously performed during the generation of a pulse, so as to determine various pulse characteristics. Also, in a number of the currently-known systems, the determination of systolic, diastolic and mean arterial blood pressures is typically performed in a "batch" type mode. That is, sampled data is obtained throughout the cuff deflation cycle from a pressure above systolic blood pressure to a pressure below diastolic blood pressure. During this pressure deflation interval, data is stored with respect to each detected pulse, including pulse characteristics and static cuff pressures associated with the occurrences of the pulses. After the entire pressure deflation cycle is completed, the stored data is utilized to analyze the pulse characteristics of the pulses and to determine systolic, diastolic and mean blood pressures.